1. Field of the Invention
The invention relates to charge injection devices optimized for IR sensing and more particularly to an improved readout circuit for a linear IR sensing array.
2. Prior Art
IR sensing CID arrays are well known, the sensor substrate material often being Indium Antimonide (InSb) or mercury cadmium telluride (HgCdTe). These materials are compound semiconductors which are doped to achieve a desired impurity level. When exposed to IR, photon collisions create hole-electron pairs in the substrate. In the usual construction, an electrode is applied to the under surface of the substrate, and an oxide layer is applied to the upper surface of the substrate, followed by a transparent electrode connected with a particular sensor element of the array on the upper surface of the oxide. The sensor element, with its insulated electrode, when suitably reversely biased, stores IR induced charges (the holes) in a potential well. If the reverse bias is removed, the charges are injected into the substrate emptying the well. Readings reflecting the IR induced charge on the sensor may be taken before and after injection if one wishes to measure the IR intensity. In practical devices, there are a large number of elements, as in a linear IR array, and a large number of readings are taken in a relatively short time. In practice, each reading of a sensor element is a double sample, and the readout process requires that the charge stored in the potential well be "injected" into the substrate.
In direct charge injection, to which the present invention has application, injection is incomplete, and the uninjected charge introduces several deleterious consequences in the output signal of the CID. While the basis for incomplete injection is more completely dealt with subsequently, one may say that it arises from the nature of the injection process. During IR exposure, IR induced charges are stored in the potential well of the sensor elements of the array. The sensor elements each have a capacitance typically one to ten picofarads. The IR induced charge, when derived from the sensor element is coupled to a node to which other sensor elements are coupled on occasion, and to which the input of a common preamplifier is connected. The preamplifier node also possesses a capacitance usually of several picofarads. In the direct charge injection process, the sensor element and node are preset to different voltages, the sensor element to a larger negative voltage which facilitates storage of IR induced charge, and the preamplifier node to a voltage which--when the two are interconnected, is sufficiently positive to inject the charges out of the well and into the substrate. When the interconnected circuitry reaches an equilibrium state, the voltage at both the sensor and the preamplifier node are equal. At the same time, the IR induced charge is found to be not all injected. A portion of IR induced charge, roughly proportional to the ratio of the capacitance of the sensor element to the sum of the capacitances of the sensor element and the preamplifier node, remains in the sensor element. In the example the proportion of retained charge to IR induced charge, which is called readout circuit lag (alpha.sub.R) can be as high as 50%.
There are several adverse effects from readout circuit lag. One adverse effect is reduced transient response for an individual sensor, and in the array a reduction of angular resolution. Another adverse affect is the tendency to emphasize larger duration IR background, an effect which on occasion drives the sensor element into saturation and restricts its dynamic range.